Glass window modules commonly used in the automotive industry typically comprise a sheet of glass, often bent into a curvo-planar shape, an encapsulation material such as a reaction-injection-molded (RIM) polyurethane or a polyvinyl chloride (PVC) positioned around a peripheral edge of the glass, and attachment means for securing the module to a motor vehicle. Often a primer is needed to enhance the bonding between the glass and the encapsulation material.
The automotive glass used in window modules is manufactured in flat sheets. The flat sheets are cut to size and optionally a frit is applied, typically around the periphery of the glass. Frit is important for decoration and exterior styling, principally to hide from view what may be positioned behind the frit, such as, for example, attachment means for securing the window module to a motor vehicle body panel, etc. Frit is normally applied by a silk-screen process to the flat sheets of glass. The frit is applied to the glass and then heated to cure the frit and bond it to the glass. The frit used in automotive applications is typically glass in particulate form, such as a bismuth-borosilicate glass particulate, generally of controlled size distribution, with a liquid carrier, and a non-transparent additive such as an iron oxide to produce an opaque dark color and act as the covering agent.
The glass is bent into shape in accordance with techniques known to those skilled in the art, either by the traditional gravity sag method in a lehr or by the more recently developed press bend techniques wherein the glass is heated sufficiently to be formed to a contoured surface. These flat glass manufacturing and forming techniques are energy intensive and consequently have undesirable costs. Furthermore, there are limitations on how far flat glass can be bent without introducing unacceptable optical distortions and/or high stresses which might cause the glass to fail.
Plastic glazing modules have been considered as a replacement for glass window modules in automotive applications. One of the principal identified advantages is weight reduction, with correspondingly improved vehicle fuel economy. Additionally, window modules having transparent polymeric materials such as acrylics and polycarbonates allow complex bends and curvo-planar shapes impractical or unacheivable by conventional glass modules while maintaining required properties such as clear optics, low and substantially uniform stress, and reduced weight. The change from glass to plastic glazing panels however, introduces new problems such as frit application, scratch resistance and control of the molding process. One recently attempted solution for some of the problems of plastic glazing window modules is shown in U.S. Pat. No. 5,035,096 to Ohtake et al. Ohtake et al shows a synthetic resin windowpane having a window body portion and a frame portion. The frame portion is hollowed out by a gas-assist injection molding process. This design has various disadvantages. The injection molding process producing the window shape has to be carefully controlled to prevent areas of high stress from developing in the part, especially around the perimeter of the window body portion where it joins the frame portion. Such stresses can lead to optical distortions and aesthetically unappealing deformities. Also, use of a gas-assist injection molding technique can produce non-uniform wall thicknesses producing localized high stress areas which lead to optical distortions, surface dimples or other unattractive exterior deformities.
Plastic glazing modules have other problems not anticipated or identified with glass modules. Transparent plastics are susceptible to scratching and other damage, e.g., clouding resulting from prolonged exposure to ultraviolet (UV) radiation. UV stabilizers can be applied to or incorporated into the plastic glazing, as well as an abrasion resistant material or hardcoat to resist scratching. Ohtake et al suggests a hardcoat painted onto the window body portion and an opaque layer painted onto an exterior side of the frame portion to hide the hollowed out portion behind it. This is disadvantageous for at least the following reasons. First, the opaque layer on the frame portion is subjected to the elements and may be scratched off or flake off, potentially exposing the transparent substrate below. Second, the hardcoat does not cover the interior of the window module, creating the potential for scratching on the interior. Third, each surface has a different gloss or shininess: the paint on the exterior of the motor vehicle has a first gloss; the opaque layer has a second gloss; and the abrasion resistant coating has a third gloss. Three different glosses so close to one another produces an aesthetically unappealing striped appearance to the exterior of the vehicle. Fourth, the abrasion resistant material or hardcoat is typically the most expensive (by volume) material in a plastic glazing module, and a painting process can waste significant amounts of hardcoat material.
Another problem with plastic glazing modules is that traditional frit used on glass cannot be used on the non-flat surfaces demanded for current automotive window applications in that bleeding of the frit can occur. Moreover, to cure the frit the window module is heated to temperatures which can damage the plastic glazing substrate. Painting a frit-like material onto a plastic glazing substrate introduces additional problems, as the paint can be difficult to accurately control and it can be difficult to produce aesthetically acceptable patterns such as a gradual fade. Furthermore, it may be difficult to ensure proper bonding between the paint and the plastic substrate. It would be highly advantageous to produce plastic glazing for motor vehicle applications having a suitable frit-like material having an aesthetically pleasing appearance and allowing stylistic freedom in creating patterns.
Another problem with the use of transparent plastics as a replacement for glass in windows is that the tensile strength of the available transparent plastics is significantly less than glass. Uneven material thicknesses can stress the plastic glazing and produce optical distortions and aesthetically unappealing dimples on the surfaces of the module. This problem is particularly apparent near mechanisms that are embedded or otherwise secured to the transparent plastic, such as attachment means used to attach the glazing module to a motor vehicle body. In conventional automotive glass window modules attachment mechanisms often comprise adhesives at the perimeter of the glazing with a series of spaced mounting studs. If such a technique were used with a plastic glazing module, the unsupported area of the plastic glazing between the mounting studs can bow away from the adhesive bonding. Furthermore, dimples or depressions can appear on the exterior surface of plastic glazing modules employing mounting studs either directly embedded in the transparent plastic or embedded in an encapsulation material.
In view of the foregoing, it is an object of the present invention to provide a plastic glazing module of superior design. It is an additional object, at least in preferred embodiments, to provide a plastic window module of reduced cost and complexity while enhancing manufacturability. It is another object of the present invention, at least in certain preferred embodiments, to provide a plastic glazing window module that is highly reliable in operation. It is another object of the invention to provide a method for producing a plastic glazing window module.